In a distributed database system, for example, where data is distributed over several database servers instead of being located in one single server, a lookup is required to route requests for a specific data item to a matching server that stores the item. This is usually implemented using “front-end” servers, much in the same way as multi-tier web server applications. They are separated from “back-end” servers that hold actual data (or applications).
In the case of databases, pointers to the back-end database servers are stored in what is called a “lookup system” or “front-end system”. These pointers are generally of a format <key, value>, where the key is a valid search key for a back-end database, and the value is an address of a back-end server where a corresponding data set is located.
Thus, when a query, e.g. an LDAP (Lightweight Directory Access Protocol) command, is issued to the distributed database system, it is first resolved in the front-end system, and then forwarded to the correct address in the back-end system, where it can be processed.
In a mobile operator environment, several locations exist where such multi-tier approach can be used, e.g. HLR (Home Location Register), Radius (Remote Authentication Dial-In User Service), Charging, Network Management, IMS (Internet Protocol Multimedia Subsystem), and web servers.
Recently, there is a trend in mobile network database design to create only one logical database that physically runs on a multitude of servers and also unifies different types of databases, for example, HLR, Radius, OAM DB (Operation And Maintenance DataBase), NM (Network Management) DB, charging DB, etc.